In a computer tomograph (CT), various absorption profiles of the body tissue to be examined are recorded by X-ray beams which are emitted from different angular directions around and onto a patient body, and a volume model of the body tissue is reconstructed using this absorption profile and knowledge of the beam path used. Before the CT is commissioned, or even at periodic intervals, such as, for example during scheduled maintenance works, the X-ray measuring facility of the CT is calibrated in order to be able to generally correct errors in the image, which errors can occur owing to the measuring apparatus or owing to the measuring principle used.
One of the effects to be considered during calibration is what is known as beam hardening, which occurs when using polychromatic X-ray spectra, and this can lead to image artifacts (what are known as “cupping artifacts”). Beam hardening is based on the physical principle that the absorption is energy-dependent, so in the X-ray spectrum, high-energy photons of human tissue or material having similar optical properties are absorbed to a lesser extent than X-ray photons with lower energy. During propagation of the X-ray radiation through an object, such as a test body made of water or through a human body, the X-ray spectrum striking the X-ray detector therefore has a transmitted spectrum with a higher energy mean than the input spectrum owing to the stronger absorption of the low-energy components.
This beam hardening can now accordingly lead to a falsification of the absorption profiles as a function of the thickness of the body tissue examined in the respective absorption profile. In particular, a volume element of the material in the interior of a relatively large object has an apparently lower absorption coefficient than a comparable volume element at the surface of the object. During three-dimensional reconstruction artifacts can consequently occur in which, in particular, a homogenous object can be displayed by the CT as inhomogeneous.
Often the absorption profiles of test bodies of known geometry, and in particular known thickness, are therefore measured for a correction, and correction functions created herefrom for the absorption data generated by the X-ray detector. However, this means a considerable amount of effort when calculating the correction function since a relatively large number of test bodies is often required or a satisfactory correction of the beam hardening.